High-Voltage Graphite//LiNi0.5Mn1.5O4 Full Cell with Bis(fluorosulfonyl)Imide and bis(trifluoromethyl)Sulfonylimide Ionic Liquid Electrolytes

Abstract

Room-temperature ionic liquids (ILs) are a promising type of lithium-ion battery (LIB) electrolyte owing to their wide electrochemical stability windows, high thermal stability, non-volatility, and environmental friendliness. Particularly, bis(fluorosulfonyl)imide (FSI−)-based ILs are appealing due to their high ionic conductivity, low viscosity, and excellent anode compatibility. Nevertheless, the strong corrosivity of FSI− toward the Al current collector at high operating potential is a major concern and needs to be addressed. Three strategies have been adopted in this work to overcome this limitation. The effects of IL composition on the electrochemical properties of the LiNi0.5Mn1.5O4 (LNMO) cathode and graphite anode are investigated. First, Li+ fractions are modulated in the N-propyl-N-methylpyrrolidinium−FSI (PMP−FSI) IL. We find that simply increasing the LiFSI concentration (even to 3.2 m) in the PMP−FSI IL electrolyte cannot effectively inhibit Al corrosion, leading to a low charge-discharge Coulombic efficiency (CE) of the LNMO electrode. Second, increasing the bis(trifluoromethyl)sulfonylimide (TFSI−)/FSI− molar ratio leads to a reduced rate capability of the LNMO cathode and a unsatisfactory compatibility with the graphite anode. Finally, we propose a Li+/TFSI−/FSI− modulation concept to optimize the IL electrolyte. The optimal 2.4 m LiTFSI/PMP−FSI IL electrolyte minimizes the corrosion current of the Al substrate at 5 V. Moreover, the relatively high Li+ and FSI− content ensures superior charge-discharge properties of both the LNMO and graphite electrodes. A high reversible capacity of approximately 135 mAh g–1 at 0.1 C based on LNMO is verified for a graphite//LNMO full cell. This cell is able to retain ~85% of its initial capacity after 200 cycles. The obtained experimental results can guide the development of IL electrolytes for high-reliability LIBs.